After authoring a slew of 5-4 conservative opinions last week and helping to effectively kill the Voting Rights Act, Justice Samuel Alito should have been in a good mood. He wasn't.

Last Monday, a cranky Alito rolled his eyes, shook his head, and looked at the ceiling - a jarring breach of court decorum - as Justice Ruth Bader Ginsburg read a dissent from the bench. In weeks prior, the justice openly glowered and rolled his eyes at Justice Elena Kagan on two separate occasions. It's a grand court tradition for justices to get grumpy and dyspeptic during the final, toughest days of the term. But even cantankerous Justice Antonin Scalia has learned to take out his anger on cases, not people (or at least, not people present).

That's a skill Alito has yet to learn - and if the past few weeks are any indication, he certainly isn't trying. That should come as no surprise. Everyone snaps from time to time, but Alito has long stood out as the rudest, most impudent justice, broadcasting his hostility and impatience to advocates and colleagues alike. He treats lawyers like children caught in a lie, grilling them on every minor point of their argument while dismissing their logic as idiotic. He handles fellow justices like hecklers who have thoughtlessly interrupted his train of thought. His demeanor is one of gruff agitation; he constantly sounds like he would rather be somewhere else.

Most of the world first witnessed the justice's churlish tendencies during the 2010 State of the Union address, when the justice shook his head and mouthed "not true" in response to President Barack Obama's Citizens United criticism. (Alito is known to dislike Obama, who as a senator voted against his confirmation. When Obama stopped by the court a week before his inauguration, Alito was conspicuously absent.) But his bad manners were on display in the courtroom long before that. At times, Alito can look bored or frustrated, occasionally following the lead of his colleague Justice Clarence Thomas and resting his eyes for a long moment. More frequently, however, Alito will hack away at an advocate's line of reasoning with smug certitude, sighing and scowling as if to indicate that the lawyer's response is simply not worth his time.

In recent years, Alito's temperamental flare-ups have become increasingly directed at his colleagues. Court procedure dictates that the justices argue with one another through the proxy of an advocate. But Alito is entirely willing to direct his barbs directly at his colleagues. Even Scalia, Alito's usual ideological ally, isn't safe from his prickly jibes. During oral arguments for Brown v. EMA, a case involving the sale of violent video games to minors, Alito mocked Scalia's originalism by informing an advocate, "I think what Justice Scalia wants to know is what James Madison thought about video games. Did he enjoy them?" Alito was obviously amused by his own belittling quip, as was the audience: The exchange drew laughter from the crowd. Scalia, though, was clearly unamused. "No," he growled, "I want to know what James Madison thought about violence." A frequent court jester himself, Scalia still takes his own philosophy quite seriously. He doesn't take kindly to a colleague ridiculing it - especially in public.

There is, to be fair, one setting in which Alito appears to be truly happy and comfortable: speeches to far-right conservatives. During these appearances, Alito is radically different from his court persona, wearing a bemused smirk that intermittently approaches genuine geniality. And there's no reason why he shouldn't be enjoying himself: Alito's out-of-court speeches are invariably political, stuffed with barely concealed jabs at liberalism and its champions.

Supreme Court justices need not be completely apolitical. Their rulings are fundamentally intertwined with politics, so they might as well be transparent about where their partisan sympathies lie. But Alito abandons any gesture toward judicial impartiality. It is simply not behavior befitting a Supreme Court justice - but then neither is much of what Alito says and does.

On the bench, he tramples advocates' arguments with a dismissive sneer and openly cuts down his colleagues' logic. Off the bench, he unapologetically trumpets his fidelity to the Republican cause. Alito's lifetime tenure ensures that his antics will never have serious professional repercussions. But as long as his impudence continues, his presence on the court will continue to demean what was once, not too long ago, a respectable institution.

Mark Joseph Stern wrote this column for Slate.

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